The present invention relates generally to semiconductor device manufacturing, and, more particularly, to an apparatus and method for reducing contamination in immersion lithography.
Lithography is one of the most important techniques utilized in semiconductor manufacturing, and is particularly used to define patterns, such as those employed in a wiring layer patterning process or a doped-region defining process for example. A lithography process generally includes an exposure step and a development step, wherein the exposure step utilizes a light source to irradiate a photoresist layer directly or through a photomask to induce chemical reactions in exposed portions. The development step is conducted to remove the exposed portion in positive resist (or the unexposed portion in negative resist) and form photoresist patterns, thus completing the transfer of photomask patterns or virtual patterns to the resist material.
Immersion lithography (IL) is rapidly emerging as the technique of choice for printing sub-100 nm photoresist structures while still using 193 nm exposure sources. By increasing the index of refraction of the medium between the last lens element of the exposure tool and the resist-coated substrate, optical systems with high effective numerical aperture (NA>1) are enabled, and thus the printable minimum feature size for a given exposure wavelength can be reduced in accordance with the well-known Rayleigh equation. Accordingly, existing immersion lithography processes are conducted at high NA in a liquid phase environment, and thus a higher resolution is achieved. This is allowed by the presence of an immersion liquid (e.g., ultra pure water) with a refractive index (n) higher than that of air (nH2O=1.44; nair=1.0). Therefore, the dimensions of the formed IC devices can be further scaled using an immersion lithography technique.
However, one drawback associated with immersion lithography stems from the physical contact between the immersion fluid and the resist material, which can potentially lead to partial image integrity failure and contamination embedded in or below the resist. More specifically, evaporation of the immersion fluid off the resist surface on the trailing edge of the shower head during exposure can lead to the concentration of trace contaminants, which can be transferred during the subsequent processing steps and finally affect device yield and performance in a severe manner. For example, traces of colloidal silica present in the immersion fluid can be concentrated in areas where immersion fluid evaporation is verified.
In addition, the trailing edge of the water pool contained by the showerhead can easily leave behind a residual immersion fluid layer, or eventually break down into droplets of variable size, under specific scanning conditions. For example, with typical wafer stage speeds in the order of 500 mm/s, any discontinuity present on the scanned surface will affect the mechanical stability of the fluid pool and lead to the formation of fluid droplets. Similarly, a low contact angle between the immersion fluid and the scanned surface will increase the shape and size of the trailing fluid edge, thus increasing the chances of forming a residual fluid layer. Either the presence of a residual fluid layer or droplets can easily lead to the formation of defects. Extractable components from the topcoat or resist layers (e.g., oligomeric material, photoacid generator, photogenerated acid, base quencher) can be extracted by the immersion fluid and result in micromasking or watermark-like defects upon fluid drying.
Accordingly, it would be desirable to be able to reduce or eliminate altogether the contamination left behind by immersion lithography.